Sweet peas are deservedly popular and quite easy to grow. They are true hardy annuals and the plants can withstand frost. Because of this they can be sown in autumn, through winter, and in spring. If sown in spring they grow quickly and flower within a few months but tend to ‘burn out’ quite quickly too as the weather gets warmer and drier (we hope).
Sowing in autumn means that the plants have time, in the cold weather, to grow slowly and develop a good root system before they are planted out in March. So the plants tend to be more vigorous, stronger and to bloom for longer, provided you prevent them setting seeds. You can sow now too. The seedlings will not be damaged by light frost and can be left outside but it is best to keep them in a coldframe or cold greenhouse to protect them from excess winter wet and from slugs and snails and from being eaten by mice.
When the seedlings are about 15cm high the tips should be pinched out because the lower sideshoots are more vigorous than the original shoot. If you are growing for showing and want flowers on long, strong stems, you can then thin these to one shoot at the time of planting and train them carefully up canes. Otherwise you can let them scramble up a support. If you do intend to grow them ‘well’ then sow one seed per pot, otherwise sow three or four seeds per pot and plant out the clump in one piece.
Like many leguminous seeds (pea family) sweet pea seeds have a tough seed coat. It prevents water from being absorbed and so can delay germination after sowing. But it is not quite that simple. The colour and hardness of the seed coat varies with the variety and with the colour of the flowers. Blue and purple sweet peas often have dark and tough seed coats while scarlet and orange often have pale and soft seed coats. It is often suggested that sweet peas should be soaked before sowing but there are problems with that because if the seeds are left in water for too long they can ‘drown’ and not grow. If you have a pack of mixed seeds there is a risk that if you soak them long enough to affect the hardest seeds the soft ones will be damaged.
It is better to ‘chit’ the seeds, scraping away part of the seed coat. Now I need to offer a disclaimer because you could cut yourself, but I have never done so, so far. Hold the seed, twixt thumb and finger, with the hilum (the stripe where the seed was attached to the pod – the belly button if you like) hidden, and chip away at the seed coat with a knife. Use the knife to drag across the seed at right angles to the blade and not along the length, so you are using a planing motion rather than a slicing action – this should reduce the risk of cutting your fingers! This way you chip off a section of seed coat.
An alternative is to scrape the seed against sandpaper but I always end up removing the ends of my fingers – useful if you have criminal intentions and want to remove your fingerprints. You may be able to hold the seeds in tweezers or a groove cut in a cork. But I favour the knife method, with the caveat that you need to be careful. Obviously you don’t need to treat pale seeds which should germinate well without any treatment. If all this sounds horribly complicated, don’t worry. If you sow the seeds as they come out of the packet they will germinate. But if there is a mix of varieties the seedlings will appear over an extended period, from two weeks to a month or more, depending on the temperature.
Which brings us to – temperature! Sweet pea seeds will grow in any temperature above 10c so you don’t need a lot of heat. They are always best when grown cool and slow in good light. The aim is not to have straggly, thin stems a metre high at planting time in late March.
When it comes to sowing, space the seeds on the compost surface and push them in about 1cm deep and keep moist.